s390/qdio: improve handling of CIWs

Fetch the individual CIWs when we actually need them, rather than
fetching both of them in qdio_setup_irq() and then needing to cache
them inside the qdio_irq.

Also deal with the error when a CIW is not available, instead of
silently dropping this error condition in qdio_setup_irq()'s caller.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>